Ondrej Jiroušek

X-ray and finite element analysis of deformation response of closed-cell metal foam subjected to compressive loading

Time-lapse X-ray computed microtomography was employed to quantify the deformation behaviour of closed-cell aluminium foam. The specimen was incrementally loaded and tomographically scanned using a custom X-ray tomographic device to capture the deforming microstructure. Because of the very small thickness of the cell walls and the high ratio between pore size and cell wall thickness cone-beam reconstruction procedure was applied. A finite element (FE) model was developed based on the reconstructed three-dimensional data. The FE model was used for two purposes: i) the nodal points were used for tracking the displacements of the deforming structure, ii) verification of the material model for description of the foams deformational behaviour. Digital volumetric correlation (DVC) algorithm was used on data obtained from the time-lapse tomography to provide a detailed description of the evolution of deformation in the complex structure of aluminium foam. The results from DVC demonstrate the possibility to use the complex microstructure of the aluminium foam as a random pattern for the correlation algorithm. The underlying FE model enables easy comparison between experimental results and results obtained from numerical simulations used for evaluation of proposed constitutive models.

Dual-energy X-ray micro-CT imaging of hybrid Ni/Al open-cell foam

In this paper, we employ dual-energy X-ray microfocus tomography (DECT) measurement to develop high-resolution finite element (FE) models that can be used for the numerical assessment of the deformation behaviour of hybrid Ni/Al foam subjected to both quasi-static and dynamic compressive loading. Cubic samples of hybrid Ni/Al open-cell foam with an edge length of [15]mm were investigated by the DECT measurement. The material was prepared using AlSi7Mg0.3 aluminium foam with a mean pore size of [0.85]mm, coated with nanocrystalline nickel (crystallite size of approx. [50]nm) to form a surface layer with a theoretical thickness of [0.075]mm. CT imaging was carried out using state-of-the-art DSCT/DECT X-ray scanner developed at Centre of Excellence Telc. The device consists of a modular orthogonal assembly of two tube-detector imaging pairs, with an independent geometry setting and shared rotational stage mounted on a complex 16-axis CNC positioning system to enable unprecedented measurement variability for highly-detailed tomographical measurements. A sample of the metal foam was simultaneously irradiated using an XWT-240-SE reflection type X-ray tube and an XWT-160-TCHR transmission type X-ray tube. An enhanced dual-source sampling strategy was used for data acquisition. X-ray images were taken using XRD1622 large area GOS scintillator flat panel detectors with an active area of [410 × 410]mm and resolution [2048 × 2048]pixels. Tomographic scanning was performed in 1,200 projections with a 0.3 degree angular step to improve the accuracy of the generated models due to the very complex microstructure and high attenuation of the investigated material. Reconstructed data was processed using a dual-energy algorithm, and was used for the development of a 3D model and voxel model of the foam. The selected parameters of the models were compared with nominal parameters of the actual foam and showed good correlation.

Hybrid Testing of Historic Materials

The paper describes hybrid approaches to testing of mechanical properties of historic materials which are mostly available in very small amounts and bulks unsuitable for standard testing procedures and techniques. Three case studies are presented. In all three, simple mechanical testing procedures were combined with special optical measurements and tailored software tools.

Radiographical investigation of fluid penetration processes in natural stones used in historical buildings

In order to ensure sustainability if historic buildings their technical state has to be inspected on regular basis. Damage assessment has to be preferably carried out using non-destructive methods otherwise damage accumulation may occur during life-cycle of the constructions. According to character of detected damage appropriate intervention measures (i.e. strengthening, consolidation, etc.) have to be then efficiently applied. Among other factors significantly influencing life span of constructions weathering agents (rain, erosion, dissolution, etc.) may cause rapid degradation of mechanical properties. In this paper X-ray radiograhical imaging was used to describe fluid penetration process in porous Maastricht limestone that is commonly used for restoration purposes. The imaging was performed in custom radiography device simulating practical in-situ measurements using microtube device. This device is a modified Karsten tube capable of determining absorbed volume and its speed even on inclined surfaces. However actual fluid penetration process in terms of saturation depth/volume ratio and shape of fluid wave propagating through microstructure is indeterminable using microtube. For this purpose real-time radiography imaging of fluid saturation process was performed to investigate behaviour of fluid in the material. Furthermore X-ray computed microtomography was performed to develop finite element model for simulation of fluid flow in the porous microstructure. Using the real-time imaging relations between penetration speed, penetration depth and penetrated volume were assessed. These results can be used to validate results from microtube measurements including nonlinear regions present when semi-spherical wave propagates through the material. Using a set of finite element simulations of the microtube experiment fluid velocity distribution in the material together with effective Darcys flux were calculated and results were compared to those from real-time imaging.

Displacement tracking in single human trabecula with metal-plated micro-spheres using X-ray radiography imaging

This study presents an improved radiographic method for strain measurement in very small samples of a single trabeculae. X-ray micro-radiography was used to track the deformation behaviour of individual trabecula during mechanical loading. As the X-ray micro-radiography images of a single trabecula show no significant features applicable for digital image correlation (DIC) a random pattern of markers was created on the surfaces of the samples to improve the accuracy of tracking. Metal plated borosilicate glassmicro-spheres (mean diameter 10 μm) were used as the markers for trabecular displacement tracking. Two different X-ray imaging setups were used for this purpose. The specimens of isolated trabeculae were loaded by a micro-mechanical testing device developed with respect to radiographical observation. This compact device enables a high precision three-point bending measurement. The specimen was continuously irradiated during the loading procedure by a micro-focus X-ray source. The radiographs were acquired by a single-photon counting silicon pixel detector and s flat panel sensor with CsI flipped scintillator plate. Circular Hough transform was used to locate positions of the spherical markers in the sequence of acquired radiographs and to calculate the strain in the loaded sample. The gold-coated micro-spheres provide clearly visible features in the sequence of radiographs after beam hardening correction, which in conjunction with pattern recognition algorithm enables to substantially improve the accuracy of strain measurements.

MODELLING ELASTO-PLASTIC BEHAVIOUR OF HUMAN SINGLE TRABECULA – COMPARISON WITH BENDING TEST

The single trabecula is basic beam-shaped element of bone structure. To improve knowledge of deformation behaviour of cancellous bone a mechanical tests at the level of single trabecula were performed. Elasto-plastic material properties (Youngs modulus and yield point) of single trabecula, assumed as an isotropic material, were assessed from three-point bending tests. Three-point bending test was simulated using Finite Element (FE) analyses and results were compared with experimental ones for determination of suitable material model for single trabecula.